As is already known, laser beams have a very high energy density. The energy density of the laser working process may easily reach at least one hundred thousand times that of an arc welding process. For example, an oxi-acetylene flame exhibits an energy density of about 10.sup.3 W/cm.sup.2 and an argon arc (200 A) exhibits an energy density of about 1.5.times.10.sup.4 W/cm.sup.2. Contrary to this, electron beams exhibit an energy density of about 10.sup.9 W/cm.sup.2 and continuous laser beams exhibit an energy density of 10.sup.9 W/cm.sup.2. In addition, the energy density of pulse lasers amounts to 10.sup.13 W/cm.sup.2. The use of beams having such a high energy density makes it possible to effect easily operations, such as surface quenching, welding, piercing or cutting, of a steel material. The surface quenching, welding or drilling of the steel material is carried out in correspondence with the energy density to be applied thereto. That is, for surface quenching, an energy density of about 1.8.times.10.sup.1 J/cm.sup.2 is used, for melting (welding), an energy density of about 1.9.times.10.sup.3 J/cm.sup.2 is used, and for vaporization (drilling, cutting), an energy density of about 4.9.times.10.sup.4 J/cm.sup.2 is used. In other words, when an energy input within a predetermined period of time is low, no melting of the steel material occurs, and, instead thereof, the surface of the steel material is quenched due to a rapid heating and cooling effect. On the other hand, as the energy input is increased, the steel material begins to fuse and, finally, the vaporization of the melted steel takes place.
Generally, when laser beams are applied at an energy density of 10.sup.6 W/cm.sup.2 onto a metal article, the surface temperature of the article increases to the evaporation temperature of the article within a period of about 1 microsecond, so as to evaporate the surface portion of the article. However, in this extremely short period of time, substantially no heat is absorbed by the inside portion of the article and substantially no increase in temperature of the inside portion of the article occurs. Also, when the surface portion of the article is vaporized away, the subsurface layer becomes a fresh surface layer of the article and, then, the fresh surface layer is evaporated away. In this manner, the article can be drilled or cut by the laser beams.
In the case where the laser beams are applied at an energy density of 10.sup.5 W/cm.sup.2 or less onto a metal article, a period of several milliseconds is necessary to elevate the surface temperature of the article to the vaporization temperature thereof. During this relatively long period of time, the under-lying layer of the article reaches a melting temperature thereof. Accordingly, the welding procedure can be accomplished by adequately controlling the time period of irradiation of the laser beams and by stopping the laser beam irradiation before the surface of the article to be vaporized.